All insects studied are specimens in the entomological museum of Northwest A&F University. More specific information is provided in Table 1.

The antennae of 10 adults of each of the four species were removed under a microscope (Nikon SMZ1500) by using sharp blades. The antennae were washed for 20 s (four times, each for 5 s) in 70% ethanol solution in an ultrasonic cleaner (KH-250DB; 15°C, 50HZ). After critical point drying, the specimens were attached to a holder using electric adhesive tape, sputter-coated with gold, examined and photographed with a S-4800 SEM (at 10 kV~15 kV).

The antennae of the four studied species of skipper butterflies are located between the compound eyes, and each consists of three components: a basal scape, pedicel, and an elongated flagellum. The first two components consist of a single short segment each one of them (Fig. 1a). The third component, the flagellum, consists of many subsegments. The typical flagellum is thin basally and becomes gradually thicker and curved, covered with scales (Fig. 1c). More types of sensilla are observed on the curved hook (Fig. 1d).

In total, six types of sensilla were observed on the antennae of these four skippers: sensilla squamiformia, sensilla trichodea, sensilla chaetica, sensilla auricillica, sensilla coeloconica, and Böhm sensilla.

Sensilla squamiformia (SQ)

This type of sensillum is scale-like and elongated with a distal end tapering, found along the base or center flagellum among the scales (Fig. 2a–d). The length of the sensilla squamiformia is 43.5±4.0 μm (Parnara guttata), 48.5±6.7 μm (Parnara bada), 47.5±5.8 μm (Pelopidas mathias), 46.3±3.8 μm (Pelopidas agna). The number of sensilla is 1–4 per flagellomere, with the terminal flagellomeres without any among the four skipper butterflies.

Figure 2.

(a) sensilla squamiformia on the flagellum of Parnara guttata (b) sensilla squamiformia of Parnara bada (c) sensilla squamiformia of Pelopidas mathias (d) sensilla squamiformia of Pelopidas agna. SQ sensilla squamiformia.

Sensilla trichodea (ST)

The sensilla trichodea are hair-like, tapering apically. They occur along the distal segments on the ventral surface (Figs 1d and 3a and b). The surface of the cuticular wall of sensilla trichodea is smooth and the wall pores are not seen with scanning electron microscope (Fig. 3c and d). These sensilla (range 27.1±3.2 μm–28±1.5 μm) are the most abundant with about 32–69 per flagellomere in the four species.

Figure 3.

(a) The sensilla chaetica, sensilla trichode and scales on the flagellum of Parnara guttata (b) the sensilla chaetica, sensilla trichode and sensilla coeloconica on the flagellum of Parnara bada (c) the sensilla chaetica and sensilla trichode on the flagellum of Pelopidas mathias (d) the sensilla chaetica and sensilla trichode on the flagellum of Pelopidas agna. Sc Scales; SCh sensilla chaetica; ST sensilla trichodea; SCo sensilla coeloconica.

Sensilla auricillica (SAu)

The sensilla auricillica are short and ear-shaped with a blunt and rounded tip. The surface of the cuticular wall of ear-shaped sensilla is covered with small pores (Fig. 4a–d). These sensilla are only scattered along the distal end of the flagellum. These sensilla (about 6–14 per flagellomere) are very similar and the length varies from 12.8±3.4 μm to 15.5±0.3 μm among all four skipper butterflies.

Figure 4.

The sensilla auricillica of Parnara guttata (a) Parnara bada (b) Pelopidas mathias (c) and Pelopidas agna (d).

Sensilla coeloconica (SCo)

The sensilla coeloconica consist of a submerged central peg with a grooved surface and blunt tip surrounded by a ring of cuticular spines (Figs 3b, 5a–d). They are found on the distal end of the flagellum (about 6–12 per flagellomere) in the four species (Fig. 1d). In Pelopidas mathias and Pelopidas agna, these sensilla are also found occasionally on the base or center of the flagellomere as they are difficult to discern since the scales will conceal them (Fig. 5c and d).

Figure 5.

The sensilla coeloconica of Parnara guttata (a) Parnara bada (b) Pelopidas mathias (c) and Pelopidas agna (d). SCo sensilla coeloconica

Sensilla squamiformia are commonly present in lepidopteran insects (Faucheux 1999). The sensilla squamiformia found in this study are similar in shape and distribution to those reported in two other butterfly species, Teinopalpus aureus Mell and Heliophorus phoenicoparyphus (Holland) (Jiang et al. 2000; Xu and Wang 2013); but the shape and distribution of these sensilla are different from several reported moth species, Synanthedon scitula (Harris), Coleophora sp. and Zamagiria dixolophella Dyar (Frank et al. 2010; Faucheux 2011; Gómez et al. 2003) and also different from Coleoptera (Hix et al. 2003; Gao et al. 2013). These aporous sensilla are inferred to have a mechanoreceptive function (Schneider 1964).

We identified only one type of sensilla trichodea among the four skipper butterflies. However, studies of other moth species have shown those sensilla can be divided into more subtypes according to their size and pore density (Faucheux 1999). For example, three subtypes of these sensilla are found on Synanthedon scitula and Ostrinia nubilalis (Hübner) (Frank et al. 2010; Hallberg et al. 1994). Furthermore, the number of these sensilla in Bombycidae, Tortricidae, Tineidae and Pyralidae is significantly different between male and female antennae (Steinbrecht 1973; George and Nagy 1984; Faucheux 1987; Wang et al. 2008). The accumulated studies have shown that multiporous sensilla trichodea are associated with olfactory reception of sex pheromones (Hansson et al. 1995; Ebbinghaus et al. 1997; Ma and Du 2000).

Sensilla chaetica found in this study are similar in structure to those reported for the Lycaenidae: Chilades pandava (Horsfield) and Heliophorus phoenicoparyphus (Jian et al. 2011; Xu and Wang 2013). These sensilla have also been observed in many other moth species, viz, Cydia nigricana (Fabricius), Bactra furfurana (Haworth), and Zamagiria dixolophella (Wall 1978; Razowski and Wojtusiak 2004; Gómez et al. 2003). Several studies noted that these uniporous sensilla to be contact chemoreceptors (Altner and Prillinger 1980; Hallberg et al. 1994).

Although the sensilla auricillica have been easily observed in the months, these sensilla on the antenna of butterfly was described for the first time in Pieris rapae L. (Faucheux 1996, 1999). Our observations on the Hesperiidae confirm their presence in the butterflies. Several studies on moth species considered multiporous sensilla auricillica as olfactory receptors for plant volatiles (Boekh et al. 1965; Kaissling 1971). Others suggest they respond to sex pheromone compounds (Ebbinghaus et al. 1997; Anderson et al. 2000; Faucheux 2006).

In this study, the multiporous sensilla coeloconica closely resemble those observed in many other Lepidoptera. This type of sensilla is considered to have a humidity and temperature sensitive function (Altner et al. 1977). Pophof (1997) reported that in Bombyx mori L., they are sensitive to plant volatiles and are possibly involved in the selection of oviposition sites. Sensilla coeloconica were found under the scales on the antennae of Pelopidas mathias and Pelopidas agna, as has not been reported in other insects.

Böhm sensilla observed here are morphologically similar to those presented in other families of Lepidoptera, e.g., Pyralidae, Tortricidae, Sesiidae (Gómez et al. 2003; Gómez and Carrasco 2008; Frank et al. 2010). The absence of dendrite in the sensillum lumen and the presence of a tubular body at the base of the hair, observed in the Böhm sensilla of Tineola bisselliella Humm. (Faucheux 1987) are characteristic of the mechanoreceptors with a proprioceptive function (Schneider 1964; Faucheux 1999).

In summary, we identified six different types of sensilla on the antennae of Parnara guttata, Parnara bada, Pelopidas mathias and Pelopidas agna. The external morphology and distribution of these sensilla among Parnara and Pelopidas, is very similar and also somewhat similar to other reported Lepidoptera. However, documents on morphology of antennal sensilla in butterfly species are still very limited yet. Further exploration on antennal sensilla of these group need merits to be conducted, which may provide useful information for taxonomy and phylogeny of Lepidoptera, and for further studies on the function of antennal sensilla and related pests control by application of sex pheromones.